1. Field of the Invention
The present invention relates to an apparatus for magneto-optically recording, reproducing and erasing data on a magneto-optical recording medium utilizing a laser beam.
2. Description of the Prior Art
When treating various kinds of data or information, recording, display and reproduction are the essential technique many techniques have been developed. For instance, there an image tube, a solid image element, a sensor array or the like are means for reading. As the display means, a cathode ray tube, optical projector, and plasma displays and a printer, such as, an electrostatic printer and an ink jet printer may be employed. As the recording means, a magnetic disc, a tape, a drum, a magnetic bubble element and a semiconductor memory may be employed.
These means for reading, recording and displaying have substantially been improved in their degree of resolution, the speed of recording, reproduction and reading or the like, due to the development of laser techniques which provide a strong and fine light beam and thus, many machines and devices utilizing such laser techniques have been developed.
Among others, an optical memory material capable of rewriting information or data has attracted interest many studies to develop magneto-optical recording materials have been done in recent years.
When recording and reproducing information on an magneto-optical disc, a static magnetic field is applied to the surface of the disc, on which a crystalline or amorphous layer of magnetic materials is deposited to act as a recording medium, to induce a perpendicular magnetized layer on the disc, the magnetization of which is oriented to the same direction due to the static magnetic field applied. Then, information or data is recorded on the surface by a laser spot while applying a weak magnetic field to a desired position of the recording medium having a direction of magnetization opposite to that of the previously magnetized layer. At this stage, the temperature of the layer partially rises over Curie point by the irradiation of a laser beam and as a result the magnetization at that point is reversed and the recording of information or data is completed. The information recorded on the magneto-optical recording disc may be reproduced by irradiating a laser beam to the surface layer of the magneto-optical disc, detecting the degree of rotation of the plane polarized due to Kerr effect or that polarized due to Faraday effect of the recording medium through an analyzer disposed in the light path, as the strength of transmitted or reflected light. The record medium generally comprises a disc of plastics or glass as the substrate and an amorphous alloy layer composed of a rare earth metal such as gadolinium (Gd), terbium (Tb), dysprosium (Dy) and holmium (Ho) and a transition metal such as iron (Fe), cobalt (Co) and nickel (Ni), for instance, Gd-Fe, Gd-Co, Tb-Fe, Tb-Gd-Fe.
Such magneto-optical materials must fulfill the following requirements:
(i) it must be a material such that the magnetization may easily be reversed at a relatively low temperature to improve the sensitivity of the recording medium during recording; PA1 (ii) it must have a rather high Kerr rotational angle to increase the S/N ratio of the reproduction signals; PA1 (iii) on the surface thereof, a perpendicular magnetized layer may be formed, which makes it possible to record information in more higher record density.
In order to carry out the magneto-optical recording of information or data, it is necessary to change the direction of the magnetic field to be applied to the disc at the time of recording and erasing information and therefore, as the means for applying magnetic field, an electromagnetic, a permanent magnet or an air-core coil is generally used. However, since the strength of the magnetic field required for recording information usually differs from that of the magnetic field required for erasing recorded information, it is necessary to change not only the direction of the current supplied to the electromagnetic and the air-core coil but the strength thereof to switch from recording to erasing and vice versa. The performance of the electromagnet and the air-core coil is generally defined by their maximum values of the generated magnetic field requirements and as a result, a large-sized electromagnet and air-core coil are needed as well as the use of a power supply having a complicated construction.
Furthermore, if a permanent magnet is used, the apparatus for recording, reproducing and erasing information or data must be provided with a driving mechanism to move the magnet and to reverse the magnetic field and this makes the construction of the apparatus quite complicated.